The present invention relates to magnetic tape transports, and more particularly, to a mechanism for insuring proper extraction and insertion of a leader block of a tape cartridge with respect to both the cartridge housing and the take-up hub.
One magnetic tape cartridge particularly suited for storing digital data is defined in Proposed American National Standard (ANSI) X3B5/87-238. This tape cartridge contains a single reel of magnetic tape including a leader block for interfacing with an automatic threading system and is hereafter referred to as the "leader block tape cartridge. " This cartridge is further illustrated and described in U.S. Pat. Nos. 4,335,858; 4,334,656; 4,383,660 and 4,452,406.
A number of tape transports have been developed that utilize the aforementioned leader block tape cartridge. One example is the IBM (Trademark) 3480 tap transport. FIG. 1 of the drawings herein illustrates a conventional mechanism for extracting the leader block, threading the tape and re-inserting the leader block back into the tape cartridge. A fragmentary, vertical section of the corner of the housing of the leader block tape cartridge is denoted with the reference numeral 10. This corner of the tape cartridge has a receptacle defined by upper and lower walls 12 and 14 for removably receiving the leader block 16 therein. This leader block is connected to the terminal end of a length of half-inch magnetic tape 18 wound about a supply reel (not illustrated) within the cartridge housing. Heretofore, each tape transport for the leader block tape cartridge has utilized a threader pin such as 20 whose narrow upper portion 20a is received in the smaller upper portion of a cylindrical aperture 22 formed in the leader block 16 upon insertion of the cartridge into the drive. The pin or cartridge is then moved vertically so that its enlarged lower portion 20b is captured in the lower enlarged portion of the aperture 22 in the leader block. Thereafter the threader pin is moved horizontally by a rotating arm such as 24 or a guide belt to pull the leader block and the tape connected thereto along a tape path (not shown) until the leader block is received in a slot formed in a take-up hub (not shown). The read/write head and take-up hub (neither illustrated in FIG. 1) are positioned above a horizontal deck plate 26 which typically forms a part of the frame of the tape drive.
One example of a leader block extraction, threading and insertion mechanism of the foregoing type is disclosed in U.S. Pat. No. 4,704,645 assigned to Cipher Data Products, Inc. See also U.S. Pat. Nos. 4,334,656; 4,335,858; 4,399,936; and 4,608,614 assigned to IBM, 4,679,747 assigned to Laser Magnetic Storage International Company and 4,742,407 assigned to Aspen Peripherals for further examples. Similar extracting, threading, and insertion arrangements for tape cartridges analogous to the leader block tape cartridge are shown in U.S. Pat. Nos. 4,477,851 and 4,646,177.
Problems have occurred with tape transports heretofore developed for the leader block tape cartridge with regard to reliable extraction and insertion of the leader block. The leader block must be extracted from the cartridge and properly inserted into the take-up hub before data can be written onto the tape and read therefrom. When the user thereafter desires to remove the tape cartridge, the leader block must be extracted from the take-up hub and then inserted into the cartridge. The positioning of the leader block must be very precise or else the insertion routines can fail. Such a failure will render the tape drive unusable for data storage and retrieval on that particular cartridge and will typically result in an expensive service call.
The foregoing problems arise from the following. The leader block 16 is relatively large and has sharp, right angle surfaces. While the walls of the leader block receptacle are tapered at 12a and 14a (FIG. 1), analysis has indicated that there is only a cumulative vertical tolerance of approximately 0.032 inches available to insure proper insertion of the leader block back into the receptacle in the corner 10 of the cartridge housing according to aforementioned proposed ANSI specification. It is apparent that if the leader block is slightly too high or too low, it will strike the forward edges of the receptacle and will not be guided into the receptacle by the tapered edges 12a and 14a. The very narrow tolerance is due in part to variations in the sizes of the leader block and the cartridge housing which are both molded of plastic material.
Heretofore it has been assumed that the tapered shoulder 22c of the threader arm would establish the height of the leader block 16 supported thereby to insure proper re-insertion. However, experience has shown this is not true because of the tolerance variations in the size of the threader pin and in the size of the aperture in the leader block. Referring to FIG. 2, leader block A has the largest aperture permissible under the ANSI standard while threader pin B is the smallest permissible under the standard. Referring to FIG. 3, leader block C has the smallest aperture permissible under the ANSI standard while threader pin D is the largest permissible under the standard. By comparing these worst case scenarios resulting from the tolerance extremes under the aforementioned ANSI standard, is readily apparent that there can be an accumulated tolerance of 0.0144 plus or minus 0.0074 inches resulting from the fabrication of the threader pin and the leader block which is in addition to another plus or minus 0.018 inches tolerance from the bottom of the leader block to the shoulder. These accumulated tolerances exceed the aforementioned vertical tolerance of 0.032 inches and lead to problems in reliably extracting the leader block from the cartridge housing and inserting the leader block back into the cartridge housing. Similar problems are experienced with regard to insertion of the leader block into the take-up hub and extraction of the leader block from the take-up hub. The cartridge uses all the available tolerance leaving none for other components (i.e. threader pin shoulder height or cartridge opening vertical placement).
Another implementation has attempted to solve the leader block extraction and insertion problem by using a threader pin whose height can be adjusted relative to its moveable support via screw threads. However, this approach requires precise factory adjustment and still doesn't adequately overcome the failures.